Biomass is often assigned a central role in future energy system scenarios as a carbon sink, making negative greenhouse gas emissions possible through carbon capture and storage of biogenic carbon dioxide from biomass-fuelled power plants. However, biomass could also serve as a strategic complement to variable renewables by supplying electricity during hours of high residual load. In this work, we investigate the role of biomass in electricity systems with net zero or negative emissions of carbon dioxide and with different levels of biomass availability. We show that access to biomass corresponding to ca. 20% of the electricity demand in primary energy terms, is of high value to the electricity system. Biomass for flexibility purposes can be a cost-efficient support to reach a carbon neutral electricity system with the main share of electricity from wind and solar power. Biomass-fired power plants equipped with carbon capture and storage in combination with natural gas combined cycle turbines are identified as being the cost-effective choice to supply the electricity system with flexibility if the availability of biomass within the electricity system is low. In contrast, in the case of excess biomass, flexibility is supplied by biomethane-fired combined cycle turbines or by biomass-fired power plants. (C) 2018 The Authors. Published by Elsevier Ltd.